skin.cpp

//Copyright (c) 2018 Ultimaker B.V.
//CuraEngine is released under the terms of the AGPLv3 or higher.

#include <cmath> // std::ceil

#include "Application.h" //To get settings.
#include "ExtruderTrain.h"
#include "skin.h"
#include "sliceDataStorage.h"
#include "settings/EnumSettings.h" //For EFillMethod.
#include "settings/types/AngleRadians.h" //For the infill support angle.
#include "settings/types/Ratio.h"
#include "utils/math.h"
#include "utils/polygonUtils.h"

#define MIN_AREA_SIZE (0.4 * 0.4)

namespace cura
{

coord_t SkinInfillAreaComputation::getSkinLineWidth(const SliceMeshStorage& mesh, const LayerIndex& layer_nr)
{
    coord_t skin_line_width = mesh.settings.get<coord_t>("skin_line_width");
    if (layer_nr == 0)
    {
        const ExtruderTrain& train_skin = mesh.settings.get<ExtruderTrain&>("top_bottom_extruder_nr");
        skin_line_width *= train_skin.settings.get<Ratio>("initial_layer_line_width_factor");
    }
    return skin_line_width;
}

coord_t SkinInfillAreaComputation::getWallLineWidth0(const SliceMeshStorage& mesh, const LayerIndex& layer_nr)
{
    coord_t wall_line_width_0 = mesh.settings.get<coord_t>("wall_line_width_0");
    if (layer_nr == 0)
    {
        const ExtruderTrain& train_wall_0 = mesh.settings.get<ExtruderTrain&>("wall_0_extruder_nr");
        wall_line_width_0 *= train_wall_0.settings.get<Ratio>("initial_layer_line_width_factor");
    }
    return wall_line_width_0;
}
coord_t SkinInfillAreaComputation::getWallLineWidthX(const SliceMeshStorage& mesh, const LayerIndex& layer_nr)
{
    coord_t wall_line_width_x = mesh.settings.get<coord_t>("wall_line_width_x");
    if (layer_nr == 0)
    {
        const ExtruderTrain& train_wall_x = mesh.settings.get<ExtruderTrain&>("wall_x_extruder_nr");
        wall_line_width_x *= train_wall_x.settings.get<Ratio>("initial_layer_line_width_factor");
    }
    return wall_line_width_x;
}
coord_t SkinInfillAreaComputation::getInfillSkinOverlap(const SliceMeshStorage& mesh, const LayerIndex& layer_nr, const coord_t& innermost_wall_line_width)
{
    coord_t infill_skin_overlap = 0;
    { // compute infill_skin_overlap
        const ExtruderTrain& train_infill = mesh.settings.get<ExtruderTrain&>("infill_extruder_nr");
        const Ratio infill_line_width_factor = (layer_nr == 0) ? train_infill.settings.get<Ratio>("initial_layer_line_width_factor") : Ratio(1.0);
        const bool infill_is_dense = mesh.settings.get<coord_t>("infill_line_distance") < mesh.settings.get<coord_t>("infill_line_width") * infill_line_width_factor + 10;
        if (!infill_is_dense && mesh.settings.get<EFillMethod>("infill_pattern") != EFillMethod::CONCENTRIC)
        {
            infill_skin_overlap = innermost_wall_line_width / 2;
        }
    }
    return infill_skin_overlap;
}

SkinInfillAreaComputation::SkinInfillAreaComputation(const LayerIndex& layer_nr, SliceMeshStorage& mesh, bool process_infill)
: layer_nr(layer_nr)
, mesh(mesh)
, bottom_layer_count(mesh.settings.get<size_t>("bottom_layers"))
, top_layer_count(mesh.settings.get<size_t>("top_layers"))
, wall_line_count(mesh.settings.get<size_t>("wall_line_count"))
, skin_line_width(getSkinLineWidth(mesh, layer_nr))
, wall_line_width_0(getWallLineWidth0(mesh, layer_nr))
, wall_line_width_x(getWallLineWidthX(mesh, layer_nr))
, innermost_wall_line_width((wall_line_count == 1) ? wall_line_width_0 : wall_line_width_x)
, infill_skin_overlap(getInfillSkinOverlap(mesh, layer_nr, innermost_wall_line_width))
, skin_inset_count(mesh.settings.get<size_t>("skin_outline_count"))
, no_small_gaps_heuristic(mesh.settings.get<bool>("skin_no_small_gaps_heuristic"))
, process_infill(process_infill)
, top_reference_wall_expansion(mesh.settings.get<coord_t>("top_skin_preshrink"))
, bottom_reference_wall_expansion(mesh.settings.get<coord_t>("bottom_skin_preshrink"))
, top_skin_expand_distance(mesh.settings.get<coord_t>("top_skin_expand_distance"))
, bottom_skin_expand_distance(mesh.settings.get<coord_t>("bottom_skin_expand_distance"))
, top_reference_wall_idx(getReferenceWallIdx(top_reference_wall_expansion))
, bottom_reference_wall_idx(getReferenceWallIdx(bottom_reference_wall_expansion))
{
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
Polygons SkinInfillAreaComputation::getWalls(const SliceLayerPart& part_here, int layer2_nr, unsigned int wall_idx)
{
    Polygons result;
    if (layer2_nr >= static_cast<int>(mesh.layers.size()))
    {
        return result;
    }
    const SliceLayer& layer2 = mesh.layers[layer2_nr];
    for (const SliceLayerPart& part2 : layer2.parts)
    {
        if (part_here.boundaryBox.hit(part2.boundaryBox))
        {
            if (wall_idx <= 0)
            {
                result.add(part2.outline);
            }
            else if (wall_idx <= part2.insets.size())
            {
                result.add(part2.insets[wall_idx - 1]); // -1 because it's a 1-based index
            }
        }
    }
    return result;
};

int SkinInfillAreaComputation::getReferenceWallIdx(coord_t& preshrink) const
{
    for (int wall_idx = wall_line_count; wall_idx > 0; wall_idx--)
    {
        coord_t wall_line_width = (wall_idx > 1)? wall_line_width_x : wall_line_width_0;
        int next_wall_idx = wall_idx - 1;
        coord_t next_wall_line_width = (next_wall_idx > 1)? wall_line_width_x : (next_wall_idx == 0)? 0 : wall_line_width_0;
        coord_t diff_to_next_wall = (wall_line_width + next_wall_line_width) / 2;
        if (std::abs(preshrink - diff_to_next_wall) <= 10)
        { // snap preshrink to closest wall
            preshrink = 0;
            return next_wall_idx;
        }
        if (preshrink < diff_to_next_wall)
        {
            return wall_idx;
        }
        preshrink -= diff_to_next_wall;
    }
    return 0;
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * generateSkinAreas reads data from mesh.layers.parts[*].insets and writes to mesh.layers[n].parts[*].skin_parts
 * generateSkinInsets only read/writes the skin_parts from the current layer.
 *
 * generateSkins therefore reads (depends on) data from mesh.layers[*].parts[*].insets and writes mesh.layers[n].parts[*].skin_parts
 */
void SkinInfillAreaComputation::generateSkinsAndInfill()
{
    generateSkinAndInfillAreas();

    SliceLayer* layer = &mesh.layers[layer_nr];
    for (unsigned int part_nr = 0; part_nr < layer->parts.size(); part_nr++)
    {
        SliceLayerPart& part = layer->parts[part_nr];
        generateSkinInsetsAndInnerSkinInfill(&part);

        generateRoofing(part);
    }
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * generateSkinAreas reads data from mesh.layers[*].parts[*].insets and writes to mesh.layers[n].parts[*].skin_parts
 */
void SkinInfillAreaComputation::generateSkinAndInfillAreas()
{
    SliceLayer& layer = mesh.layers[layer_nr];

    if (!process_infill && bottom_layer_count == 0 && top_layer_count == 0)
    {
        return;
    }

    for (unsigned int part_nr = 0; part_nr < layer.parts.size(); part_nr++)
    {
        SliceLayerPart& part = layer.parts[part_nr];

        if (part.insets.size() < wall_line_count)
        {
            continue; // the last wall is not present, the part should only get inter perimeter gaps, but no skin or infill.
        }
        generateSkinAndInfillAreas(part);
    }
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * generateSkinAreas reads data from mesh.layers[*].parts[*].insets and writes to mesh.layers[n].parts[*].skin_parts
 */
void SkinInfillAreaComputation::generateSkinAndInfillAreas(SliceLayerPart& part)
{

    Polygons original_outline = part.insets.back().offset(-innermost_wall_line_width / 2);

    // make a copy of the outline which we later intersect and union with the resized skins to ensure the resized skin isn't too large or removed completely.
    Polygons upskin;
    if (top_layer_count > 0)
    {
        upskin = Polygons(original_outline);
    }
    Polygons downskin;
    if (bottom_layer_count > 0)
    {
        downskin = Polygons(original_outline);
    }

    calculateBottomSkin(part, downskin);

    calculateTopSkin(part, upskin);

    applySkinExpansion(original_outline, upskin, downskin);

    // now combine the resized upskin and downskin
    Polygons skin = upskin.unionPolygons(downskin);

    skin.removeSmallAreas(MIN_AREA_SIZE);

    if (process_infill)
    { // process infill when infill density > 0
        // or when other infill meshes want to modify this infill
        generateInfill(part, skin);
    }

    for (PolygonsPart& skin_area_part : skin.splitIntoParts())
    {
        part.skin_parts.emplace_back();
        part.skin_parts.back().outline = skin_area_part;
    }
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
void SkinInfillAreaComputation::calculateBottomSkin(const SliceLayerPart& part, Polygons& downskin)
{
    if (static_cast<int>(layer_nr - bottom_layer_count) >= 0 && bottom_layer_count > 0)
    {
        Polygons not_air = getWalls(part, layer_nr - bottom_layer_count, bottom_reference_wall_idx).offset(bottom_reference_wall_expansion);
        if (!no_small_gaps_heuristic)
        {
            for (int downskin_layer_nr = layer_nr - bottom_layer_count + 1; downskin_layer_nr < layer_nr; downskin_layer_nr++)
            {
                not_air = not_air.intersection(getWalls(part, downskin_layer_nr, bottom_reference_wall_idx).offset(bottom_reference_wall_expansion));
            }
        }
        const double min_infill_area = mesh.settings.get<double>("min_infill_area");
        if (min_infill_area > 0.0)
        {
            not_air.removeSmallAreas(min_infill_area);
        }
        downskin = downskin.difference(not_air); // skin overlaps with the walls
    }
}

void SkinInfillAreaComputation::calculateTopSkin(const SliceLayerPart& part, Polygons& upskin)
{
    if (static_cast<int>(layer_nr + top_layer_count) < static_cast<int>(mesh.layers.size()) && top_layer_count > 0)
    {
        Polygons not_air = getWalls(part, layer_nr + top_layer_count, top_reference_wall_idx).offset(top_reference_wall_expansion);
        if (!no_small_gaps_heuristic)
        {
            for (int upskin_layer_nr = layer_nr + 1; upskin_layer_nr < layer_nr + top_layer_count; upskin_layer_nr++)
            {
                not_air = not_air.intersection(getWalls(part, upskin_layer_nr, top_reference_wall_idx).offset(top_reference_wall_expansion));
            }
        }
        // Prevent removing top skin layers
        Polygons upskin_before(upskin.difference(not_air));

        const double min_infill_area = mesh.settings.get<double>("min_infill_area");
        if (min_infill_area > 0.0)
        {
            not_air.removeSmallAreas(min_infill_area);
        }

        upskin = upskin.difference(not_air); // skin overlaps with the walls
        upskin = upskin.unionPolygons(upskin_before); // in some cases the top skin layer might be removed. To prevent it add them back
    }
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
void SkinInfillAreaComputation::applySkinExpansion(const Polygons& original_outline, Polygons& upskin, Polygons& downskin)
{
    // First we set the amount of distance we want to expand, as indicated in settings
    coord_t top_outset = top_skin_expand_distance;
    coord_t bottom_outset = bottom_skin_expand_distance;

    coord_t top_min_width = mesh.settings.get<coord_t>("min_skin_width_for_expansion") / 2;
    coord_t bottom_min_width = top_min_width;

    // Compensate for the pre-shrink applied because of the Skin Removal Width.
    // The skin removal width is satisfied by applying a close operation and
    // it's done in the calculateTopSkin and calculateBottomSkin, by expanding the infill.
    // The inset of that close operation is applied in calculateTopSkin and calculateBottomSkin
    // The outset of the close operation is applied at the same time as the skin expansion.
    top_outset += top_reference_wall_expansion;
    bottom_outset += bottom_reference_wall_expansion;

    // Calculate the shrinkage needed to fulfill the minimum skin with for expansion
    top_min_width = std::max(coord_t(0), top_min_width - top_reference_wall_expansion / 2); // if the min width is smaller than the pre-shrink then areas smaller than min_width will exist
    bottom_min_width = std::max(coord_t(0), bottom_min_width - bottom_reference_wall_expansion / 2); // if the min width is smaller than the pre-shrink then areas smaller than min_width will exist

    // skin areas are to be enlarged by skin_expand_distance but before they are expanded
    // the skin areas are shrunk by min_width so that very narrow regions of skin
    // (often caused by the model's surface having a steep incline) are not expanded

    top_outset += top_min_width; // increase the expansion distance to compensate for the min_width shrinkage
    bottom_outset += bottom_min_width; // increase the expansion distance to compensate for the min_width shrinkage

    // Execute shrinkage and expansion in the same operation
    if (top_outset)
    {
        upskin = upskin.offset(-top_min_width).offset(top_outset).unionPolygons(upskin).intersection(original_outline);
    }

    if (bottom_outset)
    {
        downskin = downskin.offset(-bottom_min_width).offset(bottom_outset).unionPolygons(downskin).intersection(original_outline);
    }
}


/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
void SkinInfillAreaComputation::generateSkinInsetsAndInnerSkinInfill(SliceLayerPart* part)
{
    for (SkinPart& skin_part : part->skin_parts)
    {
        generateSkinInsets(skin_part);
        generateInnerSkinInfill(skin_part);
    }
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
void SkinInfillAreaComputation::generateSkinInsets(SkinPart& skin_part)
{
    for (size_t inset_idx = 0; inset_idx < skin_inset_count; inset_idx++)
    {
        skin_part.insets.push_back(Polygons());
        if (inset_idx == 0)
        {
            //The 10 micron reduced inset is to prevent rounding errors from creating gaps that get filled by the fill small gaps routine.
            skin_part.insets[0] = skin_part.outline.offset(-skin_line_width / 2 + 10);
        }
        else
        {
            skin_part.insets[inset_idx] = skin_part.insets[inset_idx - 1].offset(-skin_line_width);
        }

        // optimize polygons: remove unnecessary verts
        skin_part.insets[inset_idx].simplify();
        if (skin_part.insets[inset_idx].size() < 1)
        {
            skin_part.insets.pop_back();
            return; // don't generate inner_infill areas if the innermost inset was too small
        }
    }
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
void SkinInfillAreaComputation::generateInnerSkinInfill(SkinPart& skin_part)
{
    if (skin_part.insets.empty())
    {
        skin_part.inner_infill = skin_part.outline;
        return;
    }
    const Polygons& innermost_inset = skin_part.insets.back();
    skin_part.inner_infill = innermost_inset.offset(-skin_line_width / 2);
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * generateInfill read mesh.layers[n].parts[*].{insets,skin_parts,boundingBox} and write mesh.layers[n].parts[*].infill_area
 */
void SkinInfillAreaComputation::generateInfill(SliceLayerPart& part, const Polygons& skin)
{
    if (part.insets.size() < wall_line_count)
    {
        return; // the last wall is not present, the part should only get inter perimeter gaps, but no infill.
    }
    const size_t wall_line_count = mesh.settings.get<size_t>("wall_line_count");
    const coord_t infill_line_distance = mesh.settings.get<coord_t>("infill_line_distance");

    coord_t offset_from_inner_wall = -infill_skin_overlap;
    if (wall_line_count > 0)
    { // calculate offset_from_inner_wall
        coord_t extra_perimeter_offset = 0; // to align concentric polygons across layers
        const EFillMethod fill_pattern = mesh.settings.get<EFillMethod>("infill_pattern");
        if (fill_pattern == EFillMethod::CONCENTRIC
            && infill_line_distance > mesh.settings.get<coord_t>("infill_line_width") * 2)
        {
            if (mesh.settings.get<bool>("alternate_extra_perimeter")
                && layer_nr % 2 == 0)
            { // compensate shifts otherwise caused by alternating an extra perimeter
                extra_perimeter_offset = -innermost_wall_line_width;
            }
            if (layer_nr == 0)
            { // compensate for shift caused by walls being expanded by the initial line width multiplier
                const coord_t normal_wall_line_width_0 = mesh.settings.get<coord_t>("wall_line_width_0");
                const coord_t normal_wall_line_width_x = mesh.settings.get<coord_t>("wall_line_width_x");
                const coord_t normal_walls_width = normal_wall_line_width_0 + (wall_line_count - 1) * normal_wall_line_width_x;
                const coord_t walls_width = normal_walls_width * mesh.settings.get<Ratio>("initial_layer_line_width_factor");
                extra_perimeter_offset += walls_width - normal_walls_width;
                while (extra_perimeter_offset > 0)
                {
                    extra_perimeter_offset -= infill_line_distance;
                }
            }
        }
        offset_from_inner_wall += extra_perimeter_offset - innermost_wall_line_width / 2;
    }
    Polygons infill = part.insets.back().offset(offset_from_inner_wall);

    infill = infill.difference(skin);
    infill.removeSmallAreas(MIN_AREA_SIZE);

    part.infill_area = infill.offset(infill_skin_overlap);
}

/*
 * This function is executed in a parallel region based on layer_nr.
 * When modifying make sure any changes does not introduce data races.
 *
 * this function may only read/write the skin and infill from the *current* layer.
 */
void SkinInfillAreaComputation::generateRoofing(SliceLayerPart& part)
{
    const size_t roofing_layer_count = mesh.settings.get<size_t>("roofing_layer_count");
    const size_t wall_idx = std::min(size_t(2), mesh.settings.get<size_t>("wall_line_count"));

    for (SkinPart& skin_part : part.skin_parts)
    {
        Polygons roofing;
        if (roofing_layer_count > 0)
        {
            Polygons no_air_above = getWalls(part, layer_nr + roofing_layer_count, wall_idx);
            if (!no_small_gaps_heuristic)
            {
                for (int layer_nr_above = layer_nr + 1; layer_nr_above < layer_nr + roofing_layer_count; layer_nr_above++)
                {
                    Polygons outlines_above = getWalls(part, layer_nr_above, wall_idx);
                    no_air_above = no_air_above.intersection(outlines_above);
                }
            }
            if (layer_nr > 0)
            {
                // if the skin has air below it then cutting it into regions could cause a region
                // to be wholely or partly above air and it may not be printable so restrict
                // the regions that have air above (the visible regions) to not include any area that
                // has air below (fixes https://github.com/Ultimaker/Cura/issues/2656)

                // set air_below to the skin area for the current layer that has air below it
                Polygons air_below = getWalls(part, layer_nr, wall_idx).difference(getWalls(part, layer_nr - 1, wall_idx));

                if (!air_below.empty())
                {
                    // add the polygons that have air below to the no air above polygons
                    no_air_above = no_air_above.unionPolygons(air_below);
                }
            }
            skin_part.roofing_fill = skin_part.inner_infill.difference(no_air_above);
            skin_part.inner_infill = skin_part.inner_infill.intersection(no_air_above);
        }
    }
}

void SkinInfillAreaComputation::generateInfillSupport(SliceMeshStorage& mesh)
{
    const coord_t layer_height = mesh.settings.get<coord_t>("layer_height");
    const AngleRadians support_angle = mesh.settings.get<AngleRadians>("infill_support_angle");
    const double tan_angle = tan(support_angle) - 0.01;  //The X/Y component of the support angle. 0.01 to make 90 degrees work too.
    const coord_t max_dist_from_lower_layer = tan_angle * layer_height; //Maximum horizontal distance that can be bridged.

    for (int layer_idx = mesh.layers.size() - 2; layer_idx >= 0; layer_idx--)
    {
        SliceLayer& layer = mesh.layers[layer_idx];
        SliceLayer& layer_above = mesh.layers[layer_idx + 1];

        Polygons inside_above;
        Polygons infill_above;
        for (SliceLayerPart& part_above : layer_above.parts)
        {
            inside_above.add(part_above.infill_area);
            infill_above.add(part_above.getOwnInfillArea());
        }

        for (SliceLayerPart& part : layer.parts)
        {
            const Polygons& infill_area = part.infill_area;
            if (infill_area.empty())
            {
                continue;
            }

            const Polygons unsupported = infill_area.offset(-max_dist_from_lower_layer);
            const Polygons basic_overhang = unsupported.difference(inside_above);
            const Polygons overhang_extented = basic_overhang.offset(max_dist_from_lower_layer + 50); // +50 for easier joining with support from layer above
            const Polygons full_overhang = overhang_extented.difference(inside_above);
            const Polygons infill_support = infill_above.unionPolygons(full_overhang);

            part.infill_area_own = infill_support.intersection(part.getOwnInfillArea());
        }
    }
}

void SkinInfillAreaComputation::generateGradualInfill(SliceMeshStorage& mesh)
{
    // no early-out for this function; it needs to initialize the [infill_area_per_combine_per_density]
    float layer_skip_count = 8; // skip every so many layers as to ignore small gaps in the model making computation more easy
    if (!mesh.settings.get<bool>("skin_no_small_gaps_heuristic"))
    {
        layer_skip_count = 1;
    }
    const coord_t gradual_infill_step_height = mesh.settings.get<coord_t>("gradual_infill_step_height");
    const size_t gradual_infill_step_layer_count = round_divide(gradual_infill_step_height, mesh.settings.get<coord_t>("layer_height")); // The difference in layer count between consecutive density infill areas

    // make gradual_infill_step_height divisible by layer_skip_count
    float n_skip_steps_per_gradual_step = std::max(1.0f, std::ceil(gradual_infill_step_layer_count / layer_skip_count)); // only decrease layer_skip_count to make it a divisor of gradual_infill_step_layer_count
    layer_skip_count = gradual_infill_step_layer_count / n_skip_steps_per_gradual_step;
    const size_t max_infill_steps = mesh.settings.get<size_t>("gradual_infill_steps");

    const LayerIndex min_layer = mesh.settings.get<size_t>("bottom_layers");
    const LayerIndex max_layer = mesh.layers.size() - 1 - mesh.settings.get<size_t>("top_layers");

    for (LayerIndex layer_idx = 0; layer_idx < static_cast<LayerIndex>(mesh.layers.size()); layer_idx++)
    { // loop also over layers which don't contain infill cause of bottom_ and top_layer to initialize their infill_area_per_combine_per_density
        SliceLayer& layer = mesh.layers[layer_idx];

        for (SliceLayerPart& part : layer.parts)
        {
            assert(part.infill_area_per_combine_per_density.size() == 0 && "infill_area_per_combine_per_density is supposed to be uninitialized");

            const Polygons& infill_area = part.getOwnInfillArea();

            if (infill_area.size() == 0 || layer_idx < min_layer || layer_idx > max_layer)
            { // initialize infill_area_per_combine_per_density empty
                part.infill_area_per_combine_per_density.emplace_back(); // create a new infill_area_per_combine
                part.infill_area_per_combine_per_density.back().emplace_back(); // put empty infill area in the newly constructed infill_area_per_combine
                // note: no need to copy part.infill_area, cause it's the empty vector anyway
                continue;
            }
            Polygons less_dense_infill = infill_area; // one step less dense with each infill_step
            for (size_t infill_step = 0; infill_step < max_infill_steps; infill_step++)
            {
                LayerIndex min_layer = layer_idx + infill_step * gradual_infill_step_layer_count + static_cast<size_t>(layer_skip_count);
                LayerIndex max_layer = layer_idx + (infill_step + 1) * gradual_infill_step_layer_count;

                for (float upper_layer_idx = min_layer; upper_layer_idx <= max_layer; upper_layer_idx += layer_skip_count)
                {
                    if (upper_layer_idx >= mesh.layers.size())
                    {
                        less_dense_infill.clear();
                        break;
                    }
                    const SliceLayer& upper_layer = mesh.layers[static_cast<size_t>(upper_layer_idx)];
                    Polygons relevent_upper_polygons;
                    for (const SliceLayerPart& upper_layer_part : upper_layer.parts)
                    {
                        if (!upper_layer_part.boundaryBox.hit(part.boundaryBox))
                        {
                            continue;
                        }
                        relevent_upper_polygons.add(upper_layer_part.getOwnInfillArea());
                    }
                    less_dense_infill = less_dense_infill.intersection(relevent_upper_polygons);
                }
                if (less_dense_infill.size() == 0)
                {
                    break;
                }
                // add new infill_area_per_combine for the current density
                part.infill_area_per_combine_per_density.emplace_back();
                std::vector<Polygons>& infill_area_per_combine_current_density = part.infill_area_per_combine_per_density.back();
                const Polygons more_dense_infill = infill_area.difference(less_dense_infill);
                infill_area_per_combine_current_density.push_back(more_dense_infill);
            }
            part.infill_area_per_combine_per_density.emplace_back();
            std::vector<Polygons>& infill_area_per_combine_current_density = part.infill_area_per_combine_per_density.back();
            infill_area_per_combine_current_density.push_back(infill_area);
            part.infill_area_own = nullptr; // clear infill_area_own, it's not needed any more.
            assert(part.infill_area_per_combine_per_density.size() != 0 && "infill_area_per_combine_per_density is now initialized");
        }
    }
}

void SkinInfillAreaComputation::combineInfillLayers(SliceMeshStorage& mesh)
{
    if (mesh.layers.empty() || mesh.layers.size() - 1 < static_cast<size_t>(mesh.settings.get<size_t>("top_layers")) || mesh.settings.get<coord_t>("infill_line_distance") == 0) //No infill is even generated.
    {
        return;
    }

    const coord_t layer_height = mesh.settings.get<coord_t>("layer_height");
    const size_t amount = std::max(1U, round_divide(mesh.settings.get<coord_t>("infill_sparse_thickness"), std::max(layer_height, coord_t(1)))); //How many infill layers to combine to obtain the requested sparse thickness.
    if(amount <= 1) //If we must combine 1 layer, nothing needs to be combined. Combining 0 layers is invalid.
    {
        return;
    }

    /* We need to round down the layer index we start at to the nearest
    divisible index. Otherwise we get some parts that have infill at divisible
    layers and some at non-divisible layers. Those layers would then miss each
    other. */
    LayerIndex min_layer = static_cast<LayerIndex>(mesh.settings.get<size_t>("bottom_layers") + amount) - 1;
    min_layer -= min_layer % amount; //Round upwards to the nearest layer divisible by infill_sparse_combine.
    LayerIndex max_layer = static_cast<LayerIndex>(mesh.layers.size()) - 1 - mesh.settings.get<size_t>("top_layers");
    max_layer -= max_layer % amount; //Round downwards to the nearest layer divisible by infill_sparse_combine.
    for(LayerIndex layer_idx = min_layer; layer_idx <= max_layer; layer_idx += amount) //Skip every few layers, but extrude more.
    {
        SliceLayer* layer = &mesh.layers[layer_idx];
        for(size_t combine_count_here = 1; combine_count_here < amount; combine_count_here++)
        {
            if(layer_idx < static_cast<LayerIndex>(combine_count_here))
            {
                break;
            }

            LayerIndex lower_layer_idx = layer_idx - combine_count_here;
            if (lower_layer_idx < min_layer)
            {
                break;
            }
            SliceLayer* lower_layer = &mesh.layers[lower_layer_idx];
            for (SliceLayerPart& part : layer->parts)
            {
                for (unsigned int density_idx = 0; density_idx < part.infill_area_per_combine_per_density.size(); density_idx++)
                { // go over each density of gradual infill (these density areas overlap!)
                    std::vector<Polygons>& infill_area_per_combine = part.infill_area_per_combine_per_density[density_idx];
                    Polygons result;
                    for (SliceLayerPart& lower_layer_part : lower_layer->parts)
                    {
                        if (part.boundaryBox.hit(lower_layer_part.boundaryBox))
                        {

                            Polygons intersection = infill_area_per_combine[combine_count_here - 1].intersection(lower_layer_part.infill_area).offset(-200).offset(200);
                            result.add(intersection); // add area to be thickened
                            infill_area_per_combine[combine_count_here - 1] = infill_area_per_combine[combine_count_here - 1].difference(intersection); // remove thickened area from less thick layer here
                            unsigned int max_lower_density_idx = density_idx;
                            // Generally: remove only from *same density* areas on layer below
                            // If there are no same density areas, then it's ok to print them anyway
                            // Don't remove other density areas
                            if (density_idx == part.infill_area_per_combine_per_density.size() - 1)
                            {
                                // For the most dense areas on a given layer the density of that area is doubled.
                                // This means that - if the lower layer has more densities -
                                // all those lower density lines are included in the most dense of this layer.
                                // We therefore compare the most dense are on this layer with all densities
                                // of the lower layer with the same or higher density index
                                max_lower_density_idx = lower_layer_part.infill_area_per_combine_per_density.size() - 1;
                            }
                            for (size_t lower_density_idx = density_idx; lower_density_idx <= max_lower_density_idx && lower_density_idx < lower_layer_part.infill_area_per_combine_per_density.size(); lower_density_idx++)
                            {
                                std::vector<Polygons>& lower_infill_area_per_combine = lower_layer_part.infill_area_per_combine_per_density[lower_density_idx];
                                lower_infill_area_per_combine[0] = lower_infill_area_per_combine[0].difference(intersection); // remove thickened area from lower (single thickness) layer
                            }
                        }
                    }

                    infill_area_per_combine.push_back(result);
                }
            }
        }
    }
}


}//namespace cura